Anisotropic electric properties of purple membrane and their change during the photoreaction cycle

Abstract

Purple membrane suspension shows two different orientations in electric fields of different frequencies. The orientation at low frequencies (less than or equal to approximately 10 Hz), with the membrane surface perpendicular to the electric field, is due to permanent dipole moment of the membrane and the orientation at high frequencies (greater than or equal to approximately 100 Hz), with the surface parallel to the electric field, is due to induced dipole moment. By quantitative analysis of these orientations, we determined the permanent dipole moment and the polarizability. Both values varied according to the membrane size: the permanent dipole moment ranged from 500 kD to 10 MD and was proportional to the square of the diameter of the membrane. The polarizability ranged from 1 X 10(-13) to 1 X 10(-11)cm3 and was proportional to the third to fourth power of the diameter. Because the permanent dipole moment was proportional to the area of the membrane, we could determine permanent dipole moment per bacteriorhodopsin. By determining the actual membrane size under electron microscopy, we got 98 D/bacteriorhodopsin. We also concluded that the direction of the permanent dipole moment was from the cytoplasmic to the extracellular side. These values, however, were strongly dependent on the ionic strength in the medium, suggesting a screening effect due to counter ions near the membrane surface. We evaluated the screening effect and showed about a four-charge difference between the two sides of the purple membrane. Under illumination, we found that the permanent dipole moment decreased from 98 to 63 D/bacteriorhodopsin. From the best-oriented sample, we also concluded that the angle of retinal against the axis normal to the membrane surface was greater than 68.6 degrees.